This invention relates generally to memories that use phase-change materials.
Phase-change materials may exhibit at least two different states. The states may be called the amorphous and crystalline states. Transitions between these states may be selectively initiated. The states may be distinguished because the amorphous state generally exhibits higher resistivity than the crystalline state. The amorphous state involves a more disordered atomic structure and the crystalline state involves a more ordered atomic structure. Generally, any phase-change material may be utilized; however, in some embodiments, however, thin-film chalcogenide alloy materials may be particularly suitable.
The phase-change may be induced reversibly. Therefore, the memory may change from the amorphous to the crystalline state and may revert back to the amorphous state thereafter or vice versa. In effect, each memory cell may be thought of as a programmable resistor, which reversibly changes between higher and lower resistance states.
In some situations, the cell may have a large number of states. That is, because each state may be distinguished by its resistance, a number of resistance determined states may be possible allowing the storage of multiple bits of data in a single cell.
A variety of phase-change alloys are known. Generally, chalcogenide alloys contain one or more elements from column VI of the periodic table. One particularly suitable group of alloys are GeSbTe alloys.
A phase-change material may be formed within a passage or pore defined through a dielectric material. The phase-change material may be coupled to contacts on either end of the passage.
One problem that arises is that the adhesion between the dielectric material and the phase-change material may be poor. One solution to this problem is to provide an interfacial layer that promotes adhesion between the dielectric material and the phase-change material. Generally, suitable interfacial layers are conductors such as titanium.
As a result of the use of conductive interfacial layers, the possibility exists of shunting current between the contacts past phase-change material through the interfacial layer. The state of the cell may be read by passing current through the cell to determine the resistance of the phase-change material. That is, current may be passed between the contacts through the phase-change material and as a result, the resistance of the phase-change material may be determined. However, if that current is shunted past the phase-change material, the resistivity of the phase-change material may be harder to determine.
Thus, there is a need for a way to form phase-change memory cells which with suitable adherence while reducing or even avoiding shunting of current around the phase-change material.